(a) The Field of the Invention
The present invention relates to a process and an apparatus for demonomerizing polycapronamide polymer. Polycapronamide, known also as nylon 6, is normally prepared by polymerizing caprolactam.
As is known, polycapronamide is in equilibrium with caprolactam monomer in the molten phase in which the polymer is obtained after polymerization or in which it is conveyed to the extrusion spinnerets, so that at normal spinning temperatures said phase contains approximately 10% by weight of caprolactam monomer if a stable equilibrium state is allowed to be reached. When reference is made to "monomer" or to "caprolactam" in the present invention, it should be understood that oligomeric, partly even volatile, fractions may be present therein. The oligomers will therefore always be included in the denomination "monomer" or "caprolactam".
(b) The Prior Art
The separation of the monomer from the polymer has constituted for a long time and from the very beginning of the nylon 6 industry, a technical problem of significant importance. On the one hand, of course, it is desirable for economical reasons to recover said considerable amount of caprolactam and to reuse it in polymerization. On the other hand, it is necessary to eliminate the monomer or at least to reduce it to sufficiently low percentages, in the order of 2% as a maximum, to assure that the polycapronamide products, in particular the yarns, have the desired physical and mechanical characteristics. The commonly used method for demonomerizing polycapronamide consists of washing the polymer in the condition of chips and before the chips are melted to be conveyed to the spinning apparatus.
Said method has drawbacks since the chips have then to be dried, and more importantly, the caprolactam must be finally recovered from the rather diluted solutions which are obtained from washing the chips; and further, it is wholly inapplicable whenever the polymer is directly conveyed from the polymerization apparatus to the spinning, and is not previously solidified and transformed into chips, as is the usual practice when spun fiber is manufactured.
It is generally known, in this case, to extract the caprolactam monomer from the polymer by evaporation and then to recover it by dissolving or condensing it. However no process and apparatus have been described which would permit to effect such a recovery in an industrially efficient way and as a part of an industrial spinning process. The purpose of the present invention is to provide such a process and apparatus.